Planar microwave plasma sources are widely used for materials preparation and processing and continue to attract the attention of process engineers in the semiconductor industry. Many practical designs of microwave plasma sources have been disclosed as, for example, in Kawakami et al. U.S. Pat. No. 6,399,520, and C. Tian, T. Nozawa, K. Ishibashi, H. Kameyama, and T. Morimoto, Characteristics of Large-Diameter Plasma Using a Radial-Line Slot Antenna, J. Vac. Sci. Technol. A 24(4) 2006, 1421-1424. Under conditions specific for this technology, excited plasmas involve various phenomena and chemical reactions occur in a complex plasma environment, utilizing multi-component reacting gases and flow mixtures, strongly dissociated gases, and various heating conditions. At the surface of the wafers being processed, the chemical and energetic interaction of gas species with the surfaces occurs under ion bombardment, generating synergetic effects in the interaction of the gases with the film surface.
In silicon nitride film deposition from complex molecular precursors, it has not always been understood which species are precursors for film growth. Detailed experimental analysis or accurate modeling of “surface reaction” mechanisms can provide some estimate. Some intermediate products in the bulk or at surfaces being coated may play an important role in the growth mechanism. For example, NH3 can dissociate not only in a bulk plasma but also on a fresh Si-surface, creating hydrogen and NHx fragments as well. Generally, good stoichiometric silicon nitride has been achieved at higher powers that yield good dissociation and lower silane flow, both with N2 and NH3. Disilane has not been particularly good for causing a Si and N reaction on the wafer surface. Silicon nitride growth has been shown to proceed through the nitridation of Si-rich surfaces, with key factors appearing to be excited molecular nitrogen. Studies have related to RF plasma CVD technology, but less to microwave plasmas, and none to radial line slot antenna (RLSA) plasma sources. Most relate to SiH4 but few relate to Si2H6. Delivered power is known from PECVD to have an impact on the composition of the silicon nitride films, which means that different precursors may be involved in film growth under various delivered power conditions. With a microwave frequency of 2.45 GHz, electromagnetic fields and power coupled to the processing media has played an important role. This leads to technologically significant differences between microwave plasma systems and lower-frequency RF plasma systems. Microwave plasmas involve phenomena that have not been fully understood. From process point of view, this has led to insufficient control of microwave plasma properties in materials processing.
Lack of knowledge of process conditions and hardware effects have failed to provide silicon nitride films with sufficiently uniform deposition, adequate deposition rates, and good stoichiometry.